Imaging sensors typically include a plurality of light detecting elements, also known as picture elements or pixels. The pixels are usually arranged in an array. When the array is exposed to a subject of interest, each pixel captures a certain amount of light to form an image. Imaging sensors may include non-uniformities that are inherent in the composition of the sensor. For the image to be properly viewed or evaluated, the non-uniformities should be corrected.
Each pixel in the array stores one or more values related to a characteristic of the radiation, such as color, brightness, etc., otherwise known as characteristic data. However, due to manufacturing issues, installation problems, material limitations and defects, a portion of the pixels in the array may not capture and store the characteristic data correctly. Some pixels may be considered good but may still need an adjustment to the data that is stored. Therefore, characteristic data of all the pixels in the array need to be adjusted by one or more correction components.
Generally, the array of pixels is tested and evaluated before field usage or product distribution to determine which of the correction components may need to be applied to the pixels. The correction components include a gain coefficient and an offset coefficient. The gain coefficient is a value that may be multiplied by the characteristic data in order to correct the data. In various embodiments, the gain coefficient may have a range of values from 0 to approximately 2. The offset coefficient is another value that may be added to the characteristic data to provide correction to each pixel. In various embodiments, the offset coefficient may have a range of values from approximately −512 to approximately 511.
Some pixels in the array may need their characteristic data corrected by the gain coefficients, some pixels may need correction by the offset coefficients, some pixels my need both the gain coefficient and offset coefficient corrections. If a pixel is good and needs no correction, as an example, the gain coefficient may have a value of 1 and the offset coefficient may have a value of 0.
In general, an offset coefficient and a gain coefficient are applied to every pixel in the array, so that each pixel will respond to light uniformly. This process is known as non-uniformity correction (NUC) of a pixel. A method used to calculate the NUC offset and gain coefficients involves recording flat field images at various light levels. Normally, two different light levels are selected and a pixel's response to the two different light levels is calculated as a slope and compared to the average slope of the entire sensor.
The method of calculating NUC gain coefficients is linear. Pixels, however, respond non-linearly to different light levels. The inventor discovered that less non-uniformity is observed at light levels near those used to calculate the NUC gain coefficients, but the error/non-uniformity of a non-uniformity corrected pixel may be quite dramatic at extreme ADU levels. The effect of this is that if the average video level of a scene is near the midpoint of the display range, pixels at the upper boundary of the display range may be displayed inaccurately, because the amount of gain applied is inappropriate for this video level.
Accordingly, an improved method is still needed to provide non-uniformity correction of pixels in an imaging array that overcomes the aforementioned problems.